This invention relates generally to an improved process for anisotropically increasing the growth rate of thermal oxide, and more specifically to improved methods for isolating between semiconductor devices.
In the formation of semiconductor devices, and specifically in the fabrication of semiconductor integrated circuits, an oxide layer, usually called a "field oxide", is used to isolate between adjacent devices. This is especially true with insulated gate field effect transistor circuits (MOS circuits) but is true of bipolar and other circuits as well.
Field oxide is usually formed by a process in which an oxidation resistant material such as silicon nitride or a combination of silicon nitride with silicon oxide or other materials is formed on a semiconductor substrate overlying active device regions where transistors or other devices are to be formed. The substrate is then heated in an oxidizing ambient to grow a thermal oxide on those portions of the substrate not protected by the oxidation resistant material. The nature of thermal oxide formation causes the oxide so formed to be recessed into the silicon substrate as silicon is incorporated into the silicon oxide.
There are a number of problems associated with the use of conventional process for the formation of the localized silicon oxide isolation. These problems are especially significant with newer circuits which utilize an ever increasing number of devices and in which the devices are of ever decreasing size, both in surface area and depth. To produce a thermal oxide of sufficient thickness to provide the desired electrical isolation between devices requires a significant amount of processing at an elevated temperature. Long times at elevated temperatures are inconsistent with shallow devices and tend to cause crystalline defects in the semiconductor substrate. In addition, as thermal oxides grow in thickness, they also grow laterally. Thick oxides thus use a considerable amount of lateral space and accordingly require an increase in the size of the circuit chip. This is especially true when the circuit involves a large number of devices and thus a large number of isolation regions between the devices. Still further, as the thermal oxide grows and expands laterally, the oxide encroaches under the edge of the oxidation resistant material, causing a lifting of the edge of the oxidation resistant material and forming what is known in the semiconductor industry as a "bird's beak".
Because of the foregoing and other problems associated with the conventional formation of isolating thermal oxides, there was a need for an improved process which would increase the vertical growth rate of thermal oxides, and would especially increase the vertical growth rate relative to the lateral growth rate.
Accordingly, it is an object of this invention to provide an improved process for the formation of oxide isolation between devices.
It is another object of this invention to provide an improved process for the anisotropic oxidation of silicon.